This invention relates to a method for recovering high-viscosity oil from subsurface formations, and in particular, relates to a method for reducing the viscosity of the oil by the in-situ heating of an underlying salt water formation using heat produced by heat energy radiating nuclear waste materials.
It is well known in the petroleum industry that there are vast reservoirs of petroleum materials in the earth which have not been produced because the petroleum exists in a highly viscous and waxy state such that it cannot be pumped by conventional means. Such petroleum products include those known as tar sands, oil shale and asphalt rock. As a result of this common condition, many methods have been attempted for recovering the petroleum in such deposits. Among such methods is included increasing the temperature of the petroleum in-situ in the earth formation to lower its viscosity, thereby enabling conventional production methods to be used to recover the petroleum.
Prior methods have employed chemical heating means or electrical heaters suspended within the bore holes adjacent the petroleum-bearing strata, and the passage of electric current through the formation by the use of electrodes in the plurality of adjacent wells. Additionally, gases such as CO.sub.2 have been pumped down into a well and into the oil-bearing formation to chemically combine with the oil and lower its viscosity, and in other applications the oil itself has even been ignited to produce heat and gases which would generate pressure to force the oil out of the formation into adjacent well bores while heating the viscous petroleum in the formation.
The above-mentioned prior art systems have met with only limited success for two primary reasons. First, the prior art arrangements have encountered major difficulties in supplying an adequate source of heat within the bore hole itself and ultimately to the formation over extended periods of time. Secondly, the prior art apparatus and methods for removing oil have lacked an efficient means for effectively transferring the heat from a heat source within the bore hole or limited area surrounding the bore hole to the petroleum-bearing strata itself. Such conventional heat sources as heretofore employed for these purposes are, from a thermo-dynamic standpoint, a point source of heat since the actual dimension of the heating source itself is practically negligible as compared to the size and volume of the surrounding formation.
In recent years, attempts to reduce the viscosity of the petroleum contained in subsurface strata have been directed to the use of nuclear reactions in which the greater portion of the energy released by the reactions is liberated as heat. One such method, described in U.S. Pat. No. 3,246,695, utilizes energy-radiating nuclear waste material placed within a bore hole to a point within the subsurface petroleum-bearing strata. As an added benefit, a use is found for radioactive wastes which in recent years has presented a serious disposal problem.
However, implacement of nuclear wastes within the petroleum-bearing strata must be carefully controlled in order to avoid increasing the temperature of the strata arbitrarily, thus charring the crude oil surrounding the implacement point and changing the permeability and flow characteristics of the formation. If such charring does occur, the removal of the petroleum may become even more difficult than prior to the application of the heat.
The disadvantages of the prior art, and especially the invention claimed in the afore-mentioned U.S. patent are overcome with the present invention and a method for reducing the viscosity of subsurface petroleum-bearing deposits is disclosed. More particularly, the method of the present invention permits use of higher concentrations of nuclear waste material without overheating or damaging the subsurface petroleum deposit and any problems related to possible radioactive contamination of the recovered petroleum is avoided, and a safe and convenient means is provided for disposing of spent nuclear waste material.